Field of the Invention
The present invention relates to a method for manufacturing and reinforcing a catalytic unit intended for the purification of exhaust gases, by forming a helical unit by winding from a corrugated foil band and a straight foil band disposed one on top of the other.
A catalytic unit and a method for making the unit of the type described above is known previously from, for example, GB patent 2069364. According to this method, the unit is first made from uncoated foil bands, and at both or one of the ends of the unit there are welded one or several seams the purpose of which is to prevent axial movement of the helical foil bands in relation to one another. In this method the catalytic unit cannot be coated with a support material and a catalytic material until after the welding, since the support material lowers the electric conductivity and prevents the flow of current during welding. When the support material is added to a complete unit, it does not spread evenly but accumulates in the cavities formed by the meeting points of the foils and collects in itself the actual catalyst, in which case the catalyst material will not be used effectively, a factor which for its part increases the costs and lowers the efficiency. The seams produced do not necessarily withstand the stresses the unit is subjected to during use.
DE Patent 2924592 discloses a method for making a catalytic unit, in which method an uncoated corrugated foil band and an uncoated straight foil band are secured together by brazing. This method has a disadvantage in that in connection with the brazing it is necessary to use high temperatures, which results in the weakening of the structure and a deterioration of the surface.
In this method, also, the catalytic unit cannot be coated until after the foils have been secured together. In addition, the brazing is difficult to carry out.